The presently known methods to detect and make available the current parameters concerning a conductor are normally based on the use of transducers, for example current transformers. In case of automatic low voltage circuit breakers, it is normally necessary to use at least one transducer for each phase plus one optional for the neutral. Such transducers, in particular the current transformers, are normally positioned around the conductors, whose currents have to be measured, and which constitute the primary winding of the transformer, and are operatively connected, through the corresponding secondary circuits, to the reading unit. In the case of the automatic circuit breakers, such secondary circuits are operatively connected to the protection unit of the circuit breaker itself.
The use of current transformers has the considerable advantage of the self-feeding principle. The energy that can be taken from the magnetic fields produced by the conductors exposed to measurement is in fact normally sufficient to generate useful signals for the reading and, when needed, subsequent treatments.
If from one side the current transformers can function without the necessity of an auxiliary feeding, on the other side they are in some ways limited.
A first limit is the possibility of functioning only with alternate current, and therefore they cannot be used in circuits with direct currents or with very low frequence currents.
A further limit of current transformers derives from the necessity of surrounding the conductors exposed to measurement with electromagnetic transducer means. Other problems are given by their dimensions and weight, and by the high cost of the windings, usually in copper.
A further limit of this category of transducers consists in the so called saturation phenomenon: such phenomenon is part of the nature of ferromagnetic materials, which form the windings' cores, and bring about a strong decay of the reading linearity in correspondence with high values of the currents in the conductor exposed to measurement.
In the specific case of low voltage circuit breakers, all the installation, substitution and removal procedures of current transformers are extremely complicated, and it is often necessary to remove many pieces of the device that contains them.
Different alternative solutions have been tested in order to overcome such disadvantages, but they have been scarcely used in the practice, because of other limits connected to their use.
Among the different existing alternative solutions, the Rogowski coils are to be mentioned, even though they can function only in the presence of alternate current. The Rogowsky coils, economical enough in applications characterised by strong currents, turns to be uneconomical with low currents, because their good functioning is bound to an extreme accuracy of construction and installation.
Another alternative solution already known in order to detect the values of current in electric conductors consists in the use of magnetic field sensors, for example the Hall sensors, which are very accurate, light and economical. The magnetic field sensors, which perfectly function with alternate current and with direct current, need to be fed and has a limit in the not marginal energy absorption.
In practice, in the applications characterised by low current it is impossible at the current level of the art to feed the magnetic field sensors by using the energy derived form the magnetic field present near the conductor exposed to measurement. In these cases, it is necessary an external or auxiliary feeding source. Such necessity, never overcome till now, turns into a heavy practical limit in the application of such solution.
It is evident from what described above, that at current level of the art there is the need to have systems for sensing currents in electric conductors, which represent a valid alternative to the above mentioned solutions. This necessity is much more evident in case of applications where the autonomy and the reliability of functioning are requested also with low currents, as for example in association with automatic low voltage circuit breaker.